Frédéric Lançon

On the Mossbauer effect in metallic glasses

Mossbauer experiments on amorphous materials generally show an asymmetry often attributed to quadrupole splitting effects. On the other hand, some amorphous structures can be simulated with a soft-sphere relaxed model. The authors analyse this structure from the point of view of the electric field tensor, and look at the resulting distribution law for the quadrupole splittings and the contributions of dipolar terms to the internal field. By considering further a distribution of internal fields, a very simple model for analysing Mossbauer spectra is given and discussed.

Structural description of a metallic glass model

Several atom packings have been relaxed with a Johnson potential. This mere relaxation leads to states having significant differences. A procedure is given for obtaining configurations which can be considered as representative of the same amorphous structure. To give a precise geometric description of it, the authors work out a method of decomposing the whole volume into single units: tetrahedra, octahedra and more generally deltahedra.

Superglide at an Internal Incommensurate Boundary

The intriguing possibility of frictionless gliding of one solid surface on another has been predicted for certain incommensurate interfaces in crystals, based on Aubrys solution to the Frenkel-Kontorova model of a harmonic chain in a periodic potential field. Here we test this prediction for grain boundaries by comparing atomistic simulations with direct experimental observations on the structure and load-deformation behavior of gold nanopillars containing a root-two incommensurate grain boundary. The simulations show supergliding at this boundary limited by finite-size effects which cause edges to act as defects of the incommensurate structure. Structural relaxation at the edges generates stacking faults, dislocations, and asymmetric surface steps. These features as well as the related load-displacement behavior are replicated by experimental observations on the compression of nanopillars using a quantitative nanoindentation device inside a transmission electron microscope. The good agreement between the observed and predicted behavior suggests that incommensurate interfaces could play an important role in the deformation of polycrystalline materials.

Simulation of interstitial diffusion in an amorphous structure

The authors investigate the random walk of an interstitial atom through an amorphous structure. The interaction between the diffusing particle and the atoms in the matrix is defined through a model potential which can be used in any disordered material and has strong interrelations with the corresponding Voronoi network. The stochastic process is carefully described as consisting of two random walks: one on the net of interstitial sites, the other on the time axis. Thus, the authors can perform a Monte-Carlo experiment on a simulated amorphous sample. They show that the concept of percolation greatly reduces the complexity of the problem: one can define a percolation threshold for the saddle points, such that only those which are near to this value need be considered. Numerical results are obtained which show the gaussian character of the motion. Finally, the diffusion coefficient is evaluated and shown to be a little higher than in crystalline materials.

Interface-driven phase separation in multifunctional materials: the case of GeMn ferromagnetic semiconductor

We use extensive first principle simulations to show the major role played by interfaces in the mechanism of phase separation observed in semiconductor multifunctional materials. We make an analogy with the precipitation sequence observed in over-saturated AlCu alloys, and replace the Guinier-Preston zones in this new context. A new class of materials, the

Linear scaling relaxation of the atomic positions in nanostructures

\alpha

Simulation of a reproducible model of metallic glasses by hardsphere relaxation

phases, is proposed to understand the formation of the coherent precipitates observed in the GeMn system. The interplay between formation and interface energies is analyzed for these phases and for the structures usually considered in the literature. The existence of the alpha phases is assessed with both theoretical and experimental arguments.

Strain and correlation of self-organized Ge1 xMnx nanocolumns embedded in Ge (001)

We present a method to determine the equilibrium geometry of large atomistic systems with linear scaling. It is based on a separate treatment of long and short wavelength components of the forces. While the rapidly varying part is handled by conventional methods, the treatment of the slowly varying part is based on elasticity theory. As illustrated by numerical examples containing up to a million atoms this method allows an efficient relaxation of large nanostructures.

Stress enhanced self-diffusion in Si: Entropy effect in anisotropic elastic environment

The authors show that a numerical calculation to account for the structure of amorphous metallic materials needs a careful definition of the parameters used: radial distribution function, density, mean energy per atom. As far as such parameters are considered to be characteristic of the amorphous structure, they establish a reproducible model does exist, obtained by relaxation, in a Lennard-Jones potential, of different hard-sphere packings.

THREE MYSTERIES IN SURFACE SCIENCE

We report on the structural properties of Ge_(1-x)Mn_x layers grown by molecular beam epitaxy. In these layers, nanocolumns with a high Mn content are embedded in an almost-pure Ge matrix. We have used grazing-incidence X-ray scattering, atomic force and transmission electron microscopy to study the structural properties of the columns. We demonstrate how the elastic deformation of the matrix (as calculated using atomistic simulations) around the columns, as well as the average inter-column distance can account for the shape of the diffusion around Bragg peaks.